The vectors $$\vec{u} = \left[\begin{array}{c} 1\cr 6\cr 8 \end{array}\right], \ \ \ \ \vec{v} = \left[\begin{array}{c} -3\cr -6\cr -21+k\cr \end{array}\right], \ \ \ \ \vec{w} = \left[\begin{array}{c} -3\cr -9\cr -15 \end{array}\right]$$ are linearly independent if and only if $k \ne$ .